10. Pittsburgh/San Diego

By Bryan WalshThursday, Sept. 29, 2011

Joel Sartore / National Geographic / Getty Images

During its industrial heyday, Pittsburgh was an environmental catastrophe, a steel-mill city where the soot was visible on residents' windowsills. Over the years, Pittsburgh has gone from a city of steel workers to one of middle managers  though, thankfully, the Steelers haven't gone away  and the air has improved. But pollution is still a major problem  an estimated 47,000 children in the greater Pittsburgh area suffer from asthma. Meanwhile, most Americans probably think of San Diego as a Southern California paradise. But heavy traffic on its highways  as well as localized pollution from the city's busy port  can turn the air brown, especially during rush hour. San Diego averages nearly 30 days of unhealthy ozone levels each year  though like Pittsburgh, its air has improved in recent years.